Generic Associated Types

https://rust-lang.github.io/generic-associated-types-initiat...

My sibling has the super details, but at a high level:

Rust lets you have generic types on structs and functions:

  // T is a generic type parameter, x has type T
  fn foo<T>(x: T) -> {

Rust also lets you have "associated types":

  trait MyIterator {
      type Item;

      fn next(&mut self) -> Option<Self::Item>;
  }

  struct Alternate {
       state: i32,
   }
  
  impl MyIterator for Alternate {
      // because we choose the Item type here...
      type Item = i32;
  
      // that determines the type that gets returned down here
      fn next(&mut self) -> Option<Self::Item> {

In today's Rust, `type Item` cannot be a generic type:

  // lifetimes are generics, so if you try this...
  trait MyIterator {
      type Item<'a>;

  // you'll get this error message
  error[E0658]: generic associated types are unstable

In the near future, combining these two features will be allowed.